1. Technical Field
The present invention relates to a sound system, more specifically to a sound system of a wall-mounted flat panel display apparatus.
2. Description of the Related Art
Flat panel display apparatuses such as televisions and monitors employing LCD, PDP, or OLED has been popular because of not only their clearer video quality but also their less-occupied space due to the thin thickness as compared with CRT-type display apparatuses. In particular, wall-mounted flat panel display apparatuses can be easily mounted in a wall through a simple mounting means. This can permit us to use more-widened space efficiently when we watch a TV, for example.
The sound outputted from a wall-mounted flat panel display apparatus, however, encounter several obstacles on its way. In particular, high-frequency sounds may be over-amplified or distorted by the obstacles and mid or low-frequency sounds may be weakened. This is because the wall-mounted flat panel apparatus has its own weak properties in sound output and its restriction in mounted structure. This will be described with reference to FIG. 1A through FIG. 1C.
Firstly, the weakness in the output sound is caused by the inherent problem of typical flat panel display apparatuses. Referring to FIG. 1A, a flat panel display apparatus 10 has a display area 11 for playing back a video, which occupies most of the area and volume of the whole flat panel display apparatus. This reflects the current trend of users who have some preference for wider screens.
Accordingly, in the flat display apparatus 10 there is little room for a sound system to amplify and play back the audio sound. In FIG. 1A, for example, front sound holes 13 and 13′ with speaker units behind can be placed only in only narrow remaining areas.
Moreover, the flat panel display apparatus 10 is usually very thin in its thickness from the front side 12 to a back side 14 as compared with the conventional CRT type display apparatuses. In this condition there remains only a narrow physical space in which the sound system can be mounted. The narrow ambient space may cause poor sound quality especially in the low-frequency sound component.
Secondly, the weakness in the output sound of the wall-mounted flat panel display apparatus is caused by the mounting scheme in which the flat panel display apparatus is fixed very close to the wall.
FIG. 1B shows the top view of a flat panel display apparatus mounted on a wall. In most commercial design of the flat panel display apparatus, simple speaker drivers are used for its sound system without any speaker enclosures due to the limited space available inside the flat panel display apparatus. In this case the speaker drivers are tightly attached to the surface of the apparatus case. Therefore, the output sound from drivers of the speakers 20 and 20′ can mostly propagate toward the front side 12. Here, these sounds are referred to as “front sounds.” However, the mechanical vibration of the drivers inevitably produces audio sounds propagating toward the back side 14 of the flat panel display apparatus 10. Here, these sounds are referred to as “back sounds.”
The back sounds can leave the flat panel display apparatus through holes formed at the back side 14, which may be formed for outputting the sound or other purposes such as ventilation. Then the back sounds can be reflected by the wall and directed toward the front. Since the back sounds are 180 degree out of phase with the front sounds, they may destructively interfere with the front sounds and significantly reduce the sound level. This can happen most effectively at a low-frequency sound having a long wavelength for which the path length difference between the front and the back sound of the wall mounted display apparatus produces a negligible phase change.
Thirdly, the wall mounted flat panel display apparatus can be almost hanging freely in the air because only a part of the back side 14 is in contact with the mounting bracket or post 15. This means that the flat panel display apparatus itself and its outer case are easy to vibrate mechanically by a small vibration of the wall or ambient air including audio sound waves. The mechanical vibration of the outer case, in turn, may interfere with the audio sound waves and deteriorate their spectral purity. For example, a sound wave of frequency 100 Hz represents a sinusoidal vibration of the air 100 times per second. When a mechanical vibration of the case interferes with the sound wave, the vibration of the sound wave will not be sinusoidal any more and the wave tends to contain other frequency components, from Fourier analysis, which act as noises. Unfortunately, this can happen all over the audio sound spectrum, from the high-frequency sound to the low-frequency sound, depending on the mechanical resonance of the case of the flat panel display apparatus.
When a sound wave of a certain frequency band is incident in a closed space, and the resonance condition is satisfied between the wavelength of the incident sound wave and the spatial dimension of the space, there occurs the resonance phenomenon of the sound wave, which rapidly increases the magnitude of the pertinent sound. Since the gap between the back side 14 of the flat panel display apparatus and the wall 19 in FIG. 1B is usually a few centimeters, high-frequency components of the output sound from the speakers 20 and 20′ may satisfy the resonance condition and constructively interfere in the gap. In addition, the mounting means 15 can be usually made of metallic materials. Due to its mechanical property, a small vibration of the metallic mounting means 15 can easily interfere with the resonating high-frequency sound, in the reason explained above, and may produce an amplified metallic sound that gives harshness and uneasiness to the audience.
Such interference and resonance phenomenon phenomena may be the main source of distortion and noise in the sound wave. As described above, the wall-mounted flat panel display apparatus is more vulnerable to these problems. This is shown schematically in FIG. 1C, which shows that the flat panel display apparatus 10 includes a high-frequency speaker 20a for playing back high-frequency sounds and a mid and low-frequency speaker 20b for playing back mid and low-frequency sounds.